There are many applications in which it is useful to beam energy from an energy production location to a destination at which the energy will be used. The "beamed" energy is typically microwave, or more generally energy in some radio frequency, or RF band. However, many applications for the energy require dc power. The dc power is sometimes re-converted to AC.
The conversion from microwave to dc has been done in the prior art by using a rectifying antenna, often called a rectenna. Such rectennas can take many different forms. All of these forms must convert the microwave beam into usable dc power with high efficiency. One preferred way forms a number of small rectenna units each with its own rectifying circuitry. The microwave-beamed energy is received over this area.
One particular application of this structure is in an electrically-operated aircraft which circles over a point on the ground, or navigates in the vicinity of a point on the ground. Such a microwave powered aircraft could remain in the air for many months. An appropriate converter between the microwave beam and dc power would receive the microwave beam, convert it to dc, and use that dc both to power the avionics and to power the payload. See, for example, U.S. Pat. No. 4,542,316 to Hart.
The inventors of the present invention have devised a new way to tackle both of these problems. It is an object of the present invention to provide an aperture-coupled patch element which spreads the heat dissipation to avoid heat build up and heat stress.
According to one aspect of the invention, different polarizations of microwave energy are coupled to different parts of the circuit which are physically separated from one another. Since different parts of the circuit handle different parts of the total energy load, this has the effect of spreading the power and heat dissipation among these different parts. The thermal load is hence spread to different locations on the substrate. This avoids heat build up in any one location since the production of heat is more spread out.
Another aspect of the invention relates to the efficiency of RF capture. The conversion of the RF produces many harmonic components. The non-linear elements of the rectifying circuitry, such as the diodes, produce harmonics. The application of input signals of different frequencies to a non-linear device such as a diode results in the production of mixing product frequencies that are the sums or differences of the input frequencies. The large-amplitude harmonic components that are present in any rectifier greatly increases the number and amplitude of mixing products. Not only do these diodes receive the power beam frequency, but also cellular telephone transmissions and other such transmission that could generate more harmonics in the rectifying circuit. This could cause further complex harmonics. The present invention defines structure which captures RF-beamed power and more efficiently and effectively traps the harmonic components to keep their reflection and radiation to a minimum. This is done according to the present invention by using a ground plane between the circuit and the source of microwaves; that ground plane shields against RF reflection.
The inventors of the present invention noted a number of problems with this kind of system. A first problem is heat generation. The conversion of microwave into dc causes heat to be generated. All of the components must be sized for this heat production amount. One objective of the present invention is to improve dispersal of heat, leading to lower component temperatures for any given amount of power. This is carried out according to the present invention by using an aperture-coupled patch, where two "orthogonal" polarizations of the microwave energy are coupled through different apertures to different microwave receiving elements. These "orthogonal" polarizations are any two polarizations or polarization parts that have components which are orthogonal to one another. The two elements are located in different locations on the patch. This helps to spread the thermal sources, hence reducing the thermal problem. In addition, the components are directly connected to a thermally-spreading ground plane through via holes, which even further helps to spread the thermal problem.
The inventors recognized that splitting power into two circuits not only spreads the heat problem, but also allows the use of smaller size diodes. These small size diodes can have higher impedances, and are easier to match if high power levels are desired.
The diodes introduce harmonic content to the incident microwave energy, which includes many transients and harmonics. As described above, the harmonics are multiples of the frequency of the beamed RF energy, and sums and differences of these input frequencies. The microwave beam may also be switched on and off. Additional harmonics are caused by resulting transients in the rectifying circuitry caused by any on and off switching. For ultimate efficiency we would want to trap all of the energy in these generated harmonics. This is done according to the present invention by separating the dc conversion structure from the radiating ground station by an electrically-conducting sheet, preferably a copper sheet, which forms a ground plane. This sheet prevents radiating back to the ground station.
The disclosed embodiment describes a rectenna of the aperture coupled patch type. This is often used in a power beaming system such as shown in U.S. Pat. No. 5,068,669, the disclosure of which is herein incorporated by reference. It is known to use an array of rectenna systems for receiving beaming energy. A rectenna of this type is often physically coupled to a patch antenna.
The present invention uses the patch and slot in a different way to capture the energy using techniques which are not taught or suggested by the prior art. According to the present invention, the patch acts as a collector for microwaves. The slots work in conjunction with the patch. A typical slot antenna radiates energy in both hemispheres above the slot and below the antenna of the slot. By placing the patch on one side of the slot, energy is concentrated into the hemisphere which has the patch. The slot antenna and patch together are called an aperture-coupled patch antenna and these are known in the art.
Such known patch antennas have a single slot and a single circuit associated with each patch. Moreover, all of the circuitry is usually on the same side as the patch element.
Rectenna systems prior to the present invention have also required a physical electrical connection between the patch or slot and the circuitry, and circuitry on the same side as the antenna. Many systems use a dc connection between the patch and the circuitry, and others use a so-called probe-fed patch. One problem with the probe-fed patch is that it is connected through other layers to the circuitry. Various stresses may cause the probe to separate from the rest of the patch. Such separation of the probe may constitute a failure mechanism.